Difference and sum frequency conversion in \(\chi ^{(2)}\)  materials provide an efficient and general means to access new wavelengths with established sources.
Conversion efficiency in these processes scales with field intensity, motivating resonant geometries to both increase per-unit power conversion efficiency and decrease the interaction volume and thus footprint. Resonant geometries are of particular interest for applications in quantum information in which devices must operate at the single photon level. Single photon frequency conversion processes, also called quantum frequency conversion (QFC), will enable a wider range of qubits to access telecommunication wavelengths, enabling long-distance entanglement for fiber-based quantum repeater networks [1]}, [2]}, [3]}, [4]}, [5]}, [6]}, [7]}. Further, fine control of the converted photon wavelength could be used to erase inhomogeneities in emission wavelength, permitting high-fidelity remote entanglement of inhomogeneous qubits by two-photon interference. [8]}, [9]}, [10]}.
QFC may also be useful as an upconversion mechanism for efficient room-temperature photon detection [11]}, [12]} and could provide a bridge between disparate qubit platforms [13]}.
